First Method: Shadow Behaviour
This uses Construct 2's own Shadow and Light abilities.
Add the Shadow Caster Behaviour to the walls.
And then add a Shadow Light to the scene. In the Events panel, make an event to set the position of the Light to character.x, character.y.
And now you have your first lit up zone! But the shadows are very harsh, even once you've knocked up the Light Radius, and blurred the edges a bit. And it makes the each room feel like they have impossibly high walls.